1. Technical Field of the Invention
The present invention relates to magnetic resonance imaging including non-contrast angiography, that uses a preparation scan to optimize a desired pulse sequence image quality parameter incorporated in an imaging scan in order to provide improved quality MR images.
2. Related Art
Magnetic resonance imaging (MRI) is a technique of applying a radio-frequency (RF) signal at a Larmor frequency to an object so that nuclear spins positioned in a static magnetic field are magnetically excited and then reconstructing an image from MR signals induced in response to the excitation.
However, it is not always true that an MR imaging scan is performed under the best imaging conditions. That is, an imaging scan is frequently performed under an unfavorable condition where one or more pulse sequence parameters of the imaging scan, which can significantly influence MR image of quality, has not been optimized.
One of the imaging techniques that has recently been highlighted is, for example, non-contrast MR angiography. This imaging technique provides images of blood vessels and/or flows of blood within an object, with no contrast agent administered into the object. In performing non-contrast MR angiography, a three-dimensional imaging scan is preferable when it is desired to obtain more angiographic information.
Such non-contrast MR angiography typically includes, for example, a dephase pulse to suppress a flow void phenomenon. When the flow void phenomenon is generated in response to a flow of blood, some drawbacks, such as a decrease in the intensity of an echo signal to be acquired, are caused. It is therefore preferred that the degree of such flow void phenomenon be detected beforehand, and that imaging conditions be determined for every object to be imaged in consideration of the flow void phenomenon.
In cases where non-contrast MR angiography is directed, for example, to the iliac artery, speeds of blood flowing in the iliac artery are not only different depending on individuals but also substantially different between a healthy person and a non-healthy patient. Even if the iliac artery of the same person is being imaged, speeds of blood flows change depending on which region is to be scanned.
However, from a historical viewpoint, optimization of various parameters of an imaging scan pulse sequence for such phenomena, which should be carried out prior to the imaging scan, has not been adequately studied. Hence it has been difficult for an operator to imagine the inside of an object to be examined, and then to recognize the degree of expected flow void with accuracy in a desired readout direction, before carrying out a three-dimensional scan. Instead, operators typically infer the degree of flow void using their own experience or by trial and error and then try to reflect the inferred degree into imaging conditions. A trial scan could be carried out for inferring the degree of flow void, but this trial scan would not be quantitative. Thus the total imaging time necessary for each person to be examined is, thereby a patient throughput being reduced.
In performing non-contrast MR angiography, image quality parameters of an imaging scan pulse sequence typically may include, in addition to one identifying a degree of flow void, an effective echo time TEeff, a bit indicating flow compensation, inversion recovery time, echo train spacing (ETS), the flip angle of a fat suppression pulse, an inversion time TI after application of a fat suppression pulse, the flip angle of an MT (magnetization transfer) pulse, and the flip angle of a refocusing pulse.
One conventional scan technique is known by a Japanese Patent Laid-open Publication No. 1999-239571. This reference shows an imaging scan using electrocardiographic (ECG) gating, in which a scan to measure an optimized delay time for ECG gating is proposed. This technique, however, takes only ECG gating timing into account, so that this way of scanning is far from providing other various scan parameters.